The present invention relates to low voltage RF amplifiers and mixers and to the integration thereof into a semiconductor circuit in which a single bias block may serve multiple low voltage RF circuits with compensation for both temperature and semiconductor process parameters.
With reference to the drawings, and particularly FIG. 1, radio frequency amplifiers are well known and in common use. Such amplifiers generally operate at high voltage, e.g., 12 to 15 volts, in a common emitter (i.e., a.c. grounded emitter) configuration.
With respect to prior art radio frequency amplifiers, and as shown in FIG. 1, a radio frequency input signal RF may be applied to an input terminal 20 across a coupling capacitor to the base electrode of a NPN transistor Q1. The collector electrode is connected to the bias circuit through a load resistor R.sub.L. The amplified output voltage signal may be taken from the collector electrode of the transistor Q1 and applied across a coupling capacitor to an output terminal 22.
The bias circuit for the amplifier illustrated in block form in FIG. 1 may conventionally take several forms as shown in FIGS. 2 and 3 and as disclosed in greater particularity in Heyward and DeMaw, Solid State Design For The Radio Amateur, American Radio Relay League, Inc. Newington, Conn. 1986. (See pp. 11-12, FIGS. 13 & 14) In the bias circuit of FIG. 2, temperature stability is enhanced by the use of a transistor, and in FIG. 3 by the use of an operational amplifier.
As is well known, the voltage on the collector electrode of a common emitter amplifier at zero collector current applies a constraint on the upper end of its dynamic range. Dynamic range affects interference with other frequency channels because of the overtones produced as a result of the gain compression when voltage clipping begins. As a result of the voltage drop across the bias circuit in the amplifier of FIG. 1, the maximum voltage which can be applied to the load resistor is much less then Vcc, typically about 0.6 Vcc.
More recently, and as shown, for example, in the U.S. Pat. No. 5,105,165 dated Apr. 14, 1992, it has been proposed to bias the base of a RF amplifier rather than the emitter thereof where the emitter includes an inductor. However, the bias of such amplifiers does not include means for buffering the bias circuit transistor from the effects of loading.
It is accordingly an object of the one aspect of the present invention to provide a novel low voltage RF amplifier circuit and method that obviates an important limitation on the dynamic range of the prior art in low voltage power supply applications.
It is a further object of the present invention to provide a novel amplifier and method of reducing the loss of dynamic range resulting from gain compression.
It is another object to provide a novel method and circuit for minimizing overtone interference in a RF amplifier.
It is another object of the present invention to provide a novel method and circuit to reduce gain compression in a RF amplifier without significant reduction of the noise figure.
It is yet a further object of the present invention to provide a novel amplifier and method of buffering the bias from the effects of loading.
It is yet another object of the present invention to provide a novel method and circuit to temperature compensate a low voltage amplifier.
Mixers are commonly used in RF communications to frequency upshift a low frequency information signal (e.g., an audio frequency signal) to a radio frequency for transmission, and to frequency downshift the received radio frequency signal. One single balanced active prior art mixer is illustrated in FIG. 6 where a local oscillator signal LO is mixed with a radio frequency signal RF to provide an intermediate frequency signal IF. As illustrated in FIG. 6, the bias for the mixer may be, but need not be, provided by a current source in the emitter circuit of the transistor Q3 with the gain control resistor R.sub.G.
As is well known, the noise figure of a mixer is the signal-to-noise ratio of the output signal over the signal-to-noise ratio of the input signal. The resistor R.sub.G controls the gain of the transistor Q3 and thus affects the dynamic range of the circuit. In addition, and because it is in the emitter circuit of the RF transistor Q3, it also affects the input impedance and the noise figure of the mixer.
It is accordingly an object of another aspect of the present invention to provide a novel mixer and method that obviates an important limitation on the dynamic range of prior art mixers.
It is a further object of the present invention to provide a novel circuit and method of controlling the gain of a mixer without impacting the noise figure of the circuit.
It is another object of the present invention to provide a novel method and circuit to improve a.c. stability of a mixer by reducing the gain of the transistor in the voltage-to-current converter without significantly degrading the noise figure.
In double balanced mixers, the prior art as illustrated e.g., in FIG. 8, typically locates the gain control resistor R.sub.G in the emitter circuits of the two RF amplifier transistors Q5 and Q6 and may require two complementary radio frequency input signals RF and RF. The generation of a complimentary RF input signal in turn may require the use of expensive balan input transformers.
Where two RF input signals are not desired (i.e., an unbalanced mixer), the terminal 28 in FIG. 8 may be grounded, and the IF and IF output signals combined (in a circuit not shown) with an appropriate phase shift to avoid the loss of power inherent in the non-use of an available output signal. Such combining generally results in a significant loss of bandwidth.
It is another object of the present invention to provide a circuit and method of unbalancing a mixer without significant loss of power and/or loss of bandwidth.
It is accordingly an object of another aspect of the present invention to provide a novel mixer and method that obviates the effects of gain control on the noise figure.
Base-collector capacitor compensation of the LO transistors is inherently provided by double balanced mixers such as shown in FIG. 8. Such compensation negates C.sub.u and improves the switching of the local oscillator transistors, making them more efficient and increasing the intercept point. However, in a single balanced mixer, the absence of the second pair of cross-connected transistors slows LO switching.
It is accordingly an object of the present invention to provide a novel circuit and method for compensating single balanced and unbalanced mixers.
It is also known to use a single bias block to provide bias for multiple analog circuits, as it has particular advantage in integrated circuits where lower die size, process independence and efficient use of power are desired. For example, and as shown in FIG. 13, a single bias block may comprise a power supply Vcc connected across a constant current source, two transistors and a resistor to provide a bias voltage for multiple analog voltage amplifiers in other than a common emitter or a.c. grounded emitter configuration. Examples of such low voltage bias amplifiers and ECL gates are discussed, e.g., in Gray and Meyer, Analysis And Design Of Analog Integrated Circuits, John Wiley & Sons, Inc. 1977 (See p. 236, FIG. 4.22). While such bias circuits offer immunity to injected power supply noise because the loads are differential, such differential circuits provide relatively poor noise figure performance and thus are not well suited for RF circuits.
It is accordingly an object of one aspect of the present invention to provide a novel single bias block and method that obviates many of the limitations of the prior art and the effects of a low voltage power supply on the dynamic range of plural circuits.
It is another object of the present invention to provide a novel single bias block and method of providing bias for plural common emitter circuits such as amplifiers and mixers.
As is well known, the gain of an amplifier is dependant on the operating temperature of the semiconductors in the circuit as well as process parameters. By way of example, a 1.degree. C. increase in the temperature of the integrated circuit die temperature results in a -2.2 millivolt change in the base-to-emitter voltage V.sub.BE.
The effects of .beta. and V.sub.BE mismatch are substantially reduced with emitter resistor degeneration, and degeneration improves a.c. stability and reduces gain, but degrades the noise figure. Furthermore, transistors on the same die, at the same temperature, the same bias conditions and designed to have identical characteristics may mismatch and, provide collector currents with a 30% variation due to mismatched .beta..
As shown in FIG. 13, it is also known to compensate for these parameters in circuits with resistors in the emitter circuits by use of a "mirror" circuit in which a semiconductor transistor device subject both to the same operating temperature and process parameters is used as a sensor to adjust the reference voltage applied to the operational amplifier which controls conduction of the operative circuit. However, such techniques are obviously not applicable to common emitter RF stages because of the lack of an emitter resistor and because of the importance of the noise figure.
It is accordingly an object of one aspect of the present invention to provide a novel single bias block and method that obviates many of the problems of the prior art by the use of a mirror circuit to adjust the output signal of the operational amplifier which controls the conduction of the operative circuit via its base current.
It is a further object of the present invention to provide a novel single bias block and method of providing bias for a single or multitude of common emitter amplifier stages where bias control from the base terminal is desired.
It is another object of the present invention to provide a novel single bias block and method of providing bias for a single or multitude of common emitter mixer stages where the gain of the circuit is controlled by the trimming, e.g., by laser, of a gain control resistor in the collector circuit, so that the noise figure, input impedance are not affected. It is a further object of the present invention to provide a novel single bias block and method of providing temperature compensation for plural common emitter circuits such as amplifiers and mixers.